The present invention relates to a remote time calibrating system for accurately adjusting the local time of a geostationary (or synchronous) or asynchronous satellite having a time signal generating function, the local time adjustment being made to the reference time of an earth station.
On a satellite for earth exploration or astronomic observation, it is necessary to record the time of data acquisition and to transmit the time information, together with the acquired data, to an earth station. Such a satellite usually is equipped with its own time signal generating device, which may become inconsistent with the reference time on the earth, owing to aging or some other cause. A lag of the satellite time means a lag of the time of data acquisition, which would make accurate exploration or observation impossible. It is therefore desired to calibrate the satellite time so that it can precisely match the reference time on the earth station.
By the satellite time calibration system of the prior art, first a time calibrating command is transmitted from the earth station to the satellite. Then, the command is decoded in the satellite to achieve the calibration. Where the satellite is an asynchronous type, its distance from the earth station varies from moment to moment. The time at which the calibrating command is transmitted from the earth station is set in advance. In this case, the calibrating value contained in the calibrating command should incorporate the propagation delay of the command. This delay is obtained by forecasting the distance to the satellite at the time of transmission on the basis of its orbit data, the delay of the internal command transmitter, and the time delay between the command receiver and the command decoder in the satellite.
Where the satellite is of geostationary type, the distance scarcely varies with the time. Nevertheless, a unilateral calibrating command is transmitted from the earth station to the satellite, and accordingly the transmission time of the calibrating command is precisely controlled. Also incorportated into the calibrating command is the time delay resulting from a propagating from the command encoder in the earth station to the command decoder in the satellite.
As evident from the foregoing explanation, the conventional system has the following disadvantages. The calibrating command is always unilaterally sent from the earth station to the satellite; thus, the command transmission time at the earth station has to be precisely controlled. Moreover, the calculated propagation delay from the earth station to the satellite is nothing more than a forecast, and accordingly cannot be fully accurate. This lack of accuracy is particularly conspicuous if the satellite is of an asynchronous type.
Since the transmission time of the time calibrating command is the same as the time at which the satellite time is calibrated except for the propagation delay, the calibration is accomplished within a visible period if the satellite is of an asynchronous type. Only during the visible period, can the earth station transmit to and receive from the asynchronous satellite. Since the satellite is usually collecting data during a visible period, the collected data accompanying the time data will not be continuous, resulting in inconveniences in data processing or the like.